A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to image the pattern via the projection system to the substrate, the layer of resist provided on the substrate should be in the focal plane of the projection system. Focus tests have been developed to test if a substrate is positioned correctly with respect to the focal plane, in which a test pattern provided by a test patterning device is imaged on the layer of resist. Next, a latent image of the test pattern is made visible by performing post exposure processing, for instance, a post exposure bake. After this, the width of, for instance, a line of the created pattern could be measured using, for example, a scanning electron microscope (SEM). By comparing this width with a previously obtained calibration graph (Bossung curve), the defocus can be determined. It will be understood that the width of a line is smallest in the best focus position and will become larger with increasing defocus.